The long wavelength (&lgr;>&rgr;i ion-gyro radius) density homogeneities in the solar wind that raise the stellar radio scintillation index during the onset of a high-velocity stream, may be generated by a low-frequency compressional electromagnetic mode such as the magnetosonic (MS) wave. Our investigation shows that obliquely propagating MS waves are driven unstable by the double-peaked proton distributions observed in the solar wind. We propose this as a reason for the correlation of high speed streams with enhanced scintillation. The stability at 1 AU of the observed nonequilibrium, double-peaked particle distributions emerges as a natural consequence of the suggested mechanism.